An assembly is disclosed in U.S. Pat. No. 4,299,502 in which the locking assembly includes a lock washer rotatable with the motor shaft, said washer being provided, at its periphery, with a radial recess. A detent means pivotably mounted on the print unit coacts with said recess. The detent member is spring biased towards the periphery of the lock washer and coacts with a stopping edge situated in the machine for preventing the print unit from tipping except when in the angular position determined by the recess in the washer, said position corresponding to the nesting position of the type element.